A hydrostatic transmission is known from U.S. Pat. No. 5,062,267, in which a radially outer axial piston pump fixed to a casing is arranged coaxially with a radially inner axial piston motor provided on a rotor rotatably supported in the casing, and by guiding the piston of the axial piston pump and the piston of the axial piston motor by separate swash plates, the axial piston motor, which is connected to an output shaft, is driven by a working oil discharged by the axial piston pump, which is connected to an input shaft, thus outputting the rotation of the input shaft via the output shaft at a different speed. This hydrostatic transmission has a rotary valve for switching over oil passages in response to the rotation of the rotor, the rotary valve being provided between the axial piston pump and the axial piston motor.
In an expander, etc. in which high temperature, high pressure steam is used as a working medium, in a case where, for example, a plurality of sets of operating parts, which are groups of axial piston cylinders, are provided and the outputs therefrom are combined and taken out via a common output shaft, if steam intake/discharge control means are provided so as to correspond to each of the groups of axial piston cylinders, then a plurality of intake/discharge control means are required, thus increasing the dimensions of the expander, which is a problem.